Make use of layers. Copy the deatail (use the lasso, the extract filter, masks or what ever) and then press ctrl-j (PC). This will create a new layer with the detail on. Switch to the background layer (simply click on the relevant layer in the Layers Palette). You can then use what ever method you choose, gray scale, de-sat, channel mixer etc. to convert the background to B&W. You can also of course work on the detail layer, tweeking colour saturation or what ever.
(It is actually probably best not to actually work on your orginal image. At the outset, press ctrl-j and this will create a duplicate background layer. The advantage of using layers is that can switch effects on & off and the effects of the changes made.)
Hope this helps :confused-
Regards